3,734 research outputs found
CMB Anisotropies, Cosmological Parameters and Fundamental Physics: Current Status & Perspectives
I describe briefly the Cosmic Microwave Background (hereafter CMB) physics
which explains why high accuracy observations of its spatial structure are a
unique observational tool both for the determination of the global cosmological
parameters and to constrain observationally the physics of the early universe.
I also briefly survey the many experiments which have measured the anisotropies
of the CMB and led to crucial advances in observational Cosmology. The somewhat
frantic series of new results has recently culminated with the outcome of the
WMAP satellite which confirmed earlier results, set new standards of accuracy,
and suggested that the Universe may have reionised earlier than anticipated.
Many more CMB experiments are currently taking data or being planned, with the
Planck satellite on the 2007 Horizon poised to extract all the cosmological
information in the temperature anisotropies, and foray deeply into
polarisation.Comment: To appear in the proceedings of "Where Cosmology and Fundamental
Physics Meet", 23-26 June, 2003, Marseille, Franc
The Planck mission
These lecture from the 100th Les Houches summer school on "Post-planck
cosmology" of July 2013 discuss some aspects of the Planck mission, whose prime
objective was a very accurate measurement of the temperature anisotropies of
the Cosmic Microwave Background (CMB). We announced our findings a few months
ago, on March 21, 2013. I describe some of the relevant steps we took to
obtain these results, sketching the measurement process, how we processed the
data to obtain full sky maps at 9 different frequencies, and how we extracted
the CMB temperature anisotropies map and angular power spectrum. I conclude by
describing some of the main cosmological implications of the statistical
characteristics of the CMB we found. Of course, this is a very much shortened
and somewhat biased view of the \Planck\ 2013 results, written with the hope
that it may lead some of the students to consult the original papers.Comment: 53 p.-34 fig; for spacetime consideration, the file here is not
paying justice to the actual thing; a closer approximation of it can be found
at
https://www.researchgate.net/profile/Francois_Bouchet/publication/262004262_The_Planck_Mission/file/e0b495363b042e81dd.pd
Kurtosis in Large-Scale Structure as a Constraint on Non-Gaussian Initial Conditions
We calculate the kurtosis of a large-scale density field which has undergone
weakly non-linear gravitational evolution from arbitrary non-Gaussian initial
conditions. It is well known that the weakly evolved {\twelveit skewness} is
equal to its initial value plus the term induced by gravity, which scales with
the rms density fluctuation in precisely the same way as for Gaussian initial
conditions. As in the case of skewness, the evolved {\twelveit kurtosis} is
equal to its initial value plus the contribution induced by gravity. The
scaling of this induced contribution, however, turns out to be qualitatively
different for Gaussian versus non-Gaussian initial conditions. Therefore,
measurements of the kurtosis can serve as a powerful discriminating test
between the hypotheses of Gaussian and non-Gaussian nature of primordial
density fluctuations.Comment: uuencoded compressed tar file including postscript text (17 pages)
and 2 postscript figures, submitted to MNRA
Error estimation for the MAP experiment
We report here the first full sky component separation and CMB power spectrum
estimation using a Wiener filtering technique on simulated data from the
upcoming MAP experiment, set to launch in early 2001. The simulations included
contributions from the three dominant astrophysical components expected in the
five MAP spectral bands, namely CMB radiation, Galactic dust, and synchrotron
emission. We assumed a simple homogeneous and isotropic white noise model and
performed our analysis up to a spherical harmonic multipole lmax=512 on the
fraction of the sky defined by b>20 degrees. We find that the reconstruction
errors are reasonably well fitted by a Gaussian with an rms of 24 K, but
with significant deviations in the tails. Our results further support the
predictions on the resulting CMB power spectrum of a previous estimate by
Bouchet and Gispert (1999), which entailed a number of assumptions this work
removes.Comment: 5 pages, 3 color figures, version accepted in A&A Letter
Three-Point Statistics from a New Perspective
Multipole expansion of spatial three-point statistics is introduced as a tool
for investigating and displaying configuration dependence. The novel
parametrization renders the relation between bi-spectrum and three-point
correlation function especially transparent as a set of two-dimensional Hankel
transforms. It is expected on theoretical grounds, that three-point statistics
can be described accurately with only a few multipoles. In particular, we show
that in the weakly non-linear regime, the multipoles of the reduced bispectrum,
, are significant only up to quadrupole. Moreover, the non-linear bias in
the weakly non-linear regime only affects the monopole order of these
statistics. As a consequence, a simple, novel set of estimators can be
constructed to constrain galaxy bias. In addition, the quadrupole to dipole
ratio is independent of the bias, thus it becomes a novel diagnostic of the
underlying theoretical assumptions: weakly non-linear gravity and perturbative
local bias. To illustrate the use of our approach, we present predictions based
on both power law, and CDM models. We show that the presently favoured
SDSS-WMAP concordance model displays strong ``baryon bumps'' in the 's.
Finally, we sketch out three practical techniques estimate these novel
quantities: they amount to new, and for the first time edge corrected,
estimators for the bispectrum.Comment: 5 pages 6 figures, ApL accepte
Simulations of the Microwave Sky and of its ``Observations''
Here follows a preliminary report on the construction of fake millimeter and
sub-millimeter skies, as observed by virtual instruments, e.g. the COBRA/SAMBA
mission, using theoretical modeling and data extrapolations. Our goal is to
create maps as realistic as possible of the relevant physical contributions
which may contribute to the detected signals. This astrophysical modeling is
followed by simulations of the measurement process itself by a given
instrumental configuration. This will enable a precise determination of what
can and cannot be achieved with a particular experimental configuration, and
provide a feedback on how to improve the overall design. It is a key step on
the way to define procedures for the separation of the different physical
processes in the future observed maps. Note that this tool will also prove
useful in preparing and analyzing current (\eg\ balloon borne) Microwave
Background experiments. Keywords: Cosmology -- Microwave Background
Anisotropies.Comment: 6 pages of uuencoded compressed postscript (1.2 Mb uncompressed), to
appear in the proceedings of the meeting "Far Infrared and Sub-millimeter
Space Missions in the Next Decade'', Paris, France, Eds. M. Sauvage, Space
Science Revie
Self-similarity and scaling behavior of scale-free gravitational clustering
We measure the scaling properties of the probability distribution of the
smoothed density field in -body simulations of expanding universes with
scale-free initial power-spectra, with particular attention to the predictions
of the stable clustering hypothesis. We concentrate our analysis on the ratios
, , where is the averaged -body correlation function over a cell of radius
. The behavior of the higher order correlations is studied through that
of the void probability distribution function.
As functions of , the quantities , ,
exhibit two plateaus separated by a smooth transition around . In the weakly nonlinear regime, {\bar \xi}_2 \la 1, the results are in
reasonable agreement with the predictions of perturbation theory. In the
nonlinear regime, , the function is
larger than in the weakly nonlinear regime, and increasingly so with . It
is well-fitted by the expression $S_Q= ({\bar \xi}_2/100)^{0.045(Q-2)}\
{\widetilde S}_Qn. This weak dependence on scale proves {\em a
small, but significant departure from the stable clustering predictions} at
least for n=0n=+1P_0S_Qn=-2n=-1$. In these two cases, our measurements are not accurate enough to be
discriminant.Comment: 31 pages, postscript file, figure 1 missing. Postscript file
including figure 1 available at
ftp://ftp-astro-theory.fnal.gov:/pub/Publications/Pub-95-256-
Omega from the skewness of the cosmic velocity divergence
We propose a method for measuring the cosmological density parameter
from the statistics of the divergence field, , the
divergence of peculiar velocity, expressed in units of the Hubble constant, . The velocity field is spatially smoothed over to remove strongly nonlinear effects. Assuming weakly-nonlinear
gravitational evolution from Gaussian initial fluctuations, and using
second-order perturbative analysis, we show that \propto
-\Omega^{-0.6} ^2. The constant of proportionality depends on the
smoothing window. For a top-hat of radius R and volume-weighted smoothing, this
constant is , where . If the
power spectrum is a power law, , then . A Gaussian
window yields similar results. The resulting method for measuring is
independent of any assumed biasing relation between galaxies and mass.
The method has been successfully tested with numerical simulations. A
preliminary application to real data, provided by the POTENT recovery procedure
from observed velocities favors . However, because of an
uncertain sampling error, this result should be treated as an assessment of the
feasibility of our method rather than a definitive measurement of .Comment: 16 pages + 2 figures, uuencoded postscript file, also available by
anonymous ftp from ftp.cita.utoronto.ca in directory
/cita/francis/div_skewness, CITA 94-1
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